Isotactic and syndiotactic polypropylene are among the crystalline polymers which can be characterized in terms of the stereoregularity of the polymer chain. Various stereospecific structural relationships, characterized primarily in terms of syndiotacticity and isotacticity, may be involved in the formation of stereoregular polymers for various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically-unsaturated monomers, such as C3+alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g. styrene or vinyl chloride, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g, isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.
Isotactic polypropylene is conventionally used in the production of fibers in which the polypropylene is heated and then extruded through one or more dies to produce a fiber preform which is processed by a spinning and drawing operation to produce the desired fiber product. The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula:
Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene, as shown by Formula (2), is described as follows:
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each “m” representing a “meso” diad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the polymer chain lie on alternate sides of the plane of the polymer. Using the Fisher projection formula, the structure of syndiotactic polypropylene can be shown as follows:
The corresponding syndiotactic pentad is rrrr with each r representing a racemic diad. Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. Catalysts that produce syndiotactic polypropylene are disclosed in U.S. Pat. No. 4,892,851. As disclosed there, the syndiospecific metallocene catalysts are characterized as bridged structures in which one Cp group is sterically different from the others. Specifically disclosed in the '851 patent as a syndiospecific metallocene is isopropylidene(cyclopentadienyl-1-fluorenyl) zirconium dichloride.
Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes that may be characterized by the following formula:R″(C5(R′)4)2HfQp  (4)In Formula (4), (C5(R′)4) is a cyclopentadienyl or substituted cyclopentadienyl group, R′ is independently hydrogen or a hydrocarbyl radical having 1–20 carbon atoms, and R″ is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1–20 carbon atoms and p is 2.
Metallocene catalysts, such as those described above, can be used either as so-called “neutral metallocenes” in which case an alumoxane, such as methylalumoxane, is used as a co-catalyst, or they can be employed as so-called “cationic metallocenes” which incorporate a stable non-coordinating anion and normally do not require the use of an alumoxane. For example, syndiospecific cationic metallocenes are disclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, the metallocene cation is characterized by the cationic metallocene ligand having sterically dissimilar ring structures that are joined to a positively charged coordinating transition metal atom. The metallocene cation is associated with a stable non-coordinating counter-anion. Similar relationships can be established for isospecific metallocenes.
Catalysts employed in the polymerization of alpha-olefins may be characterized as supported catalysts or as unsupported catalysts, sometimes referred to as homogeneous catalysts. Metallocene catalysts are often employed as unsupported or homogeneous catalysts, although, as described below, they also may be employed in supported catalyst components. Traditional supported catalysts are the so-called “conventional” Ziegler-Natta catalysts, such as titanium tetrachloride supported on an active magnesium dichloride, as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. A supported catalyst component, as disclosed in the Myer '718 patent, includes titanium tetrachloride supported on an “active” anhydrous magnesium dihalide, such as magnesium dichloride or magnesium dibromide. The supported catalyst component in Myer '718 is employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethylaluminum (TEAL). The Myer '717 patent discloses a similar compound that may also incorporate an electron donor compound that may take the form of various amines, phosphenes, esters, aldehydes, and alcohols.
While metallocene catalysts are generally proposed for use as homogeneous catalysts, it is also known in the art to provide supported metallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst component may be employed in the form of a supported catalyst. As described in the Welborn '432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, zirconia and the like. Non-metallocene metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The Welborn '561 patent discloses a heterogeneous catalyst that is formed by the reaction of a metallocene and an alumoxane in combination with the support material. A catalyst system embodying both a homogeneous metallocene component and a heterogeneous component, which may be a “conventional” supported Ziegler-Natta catalyst, e.g. a supported titanium tetrachloride, is disclosed in U.S. Pat. No. 5,242,876 to Shamshoum et al. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. No. 5,308,811 to Suga et al and U.S. Pat. No. 5,444,134 to Matsumoto.
The polymers normally employed in the preparation of drawn polypropylene fibers are normally prepared through the use of conventional Ziegler-Natta catalysts of the type disclosed, for example, in the aforementioned patents to Myer et al. U.S. Pat. No. 4,560,734 to Fujishita and U.S. Pat. No. 5,318,734 to Kozulla disclose the formation of fibers by heating, extruding, melt spinning, and drawing from polypropylene produced by titanium tetrachloride-based isotactic polypropylene. Particularly, as disclosed in the patent to Kozulla, the preferred isotactic polypropylene for use in forming such fibers has a relatively broad molecular weight distribution (abbreviated MWD), as determined by the ratio of the weight average molecular weight (Mw) to the number average molecular (Mn) of about 5.5 or above. Preferably, as disclosed in the Kozulla patent, the molecular weight distribution, Mw/Mn, is at least 7.
A process for the production of polypropylene fibers formed from isotactic polypropylene prepared through the use of isospecific metallocene catalysts is disclosed in U.S. Pat. No. 5,908,594 to Gownder et al. As disclosed in Gownder, the polypropylene is characterized in terms of 0.5–2% of 2-1 insertions and has an isotacticity of at least 95% meso diads. This results in intermittent head-to-head insertions to provide a polymer structure that behaves somewhat in the nature of a random ethylene/propylene copolymer. The resulting fibers have good characteristics in terms of mechanical properties and machine operation, including machine speed.
A process for the production of polypropylene fibers formed from syndiotactic polypropylene is disclosed in U.S. Pat. No. 5,272,003 to Peacock. As disclosed in Peacock, the catalyst employed in the production of the syndiotactic polypropylene can be Ziegler-Natta catalyst, such as disclosed in U.S. Pat. Nos. 3,305,538 and 3,258,455 to Natta et al, or they may be prepared through the use of syndiospecific metallocene catalysts of the type disclosed in U.S. Pat. No. 4,892,851 to Ewen et al. In Peacock, the fibers and the resulting spun yarn are characterized as partially oriented (POY) or as fully oriented (FOY). Fibers employed to make a yarn of lower orientation are described in Peacock as spun at speeds below about 1500 meters per minute whereas those spun at speeds above about 2500 meters per minute are characterized as partially oriented. Peacock discloses that syndiotactic polypropylene fibers of a low orientation, i.e. at speeds below about 1500 meters per minute, should be drawn at a high draw ratio of about 4.7 to produce fully oriented yarn. For partially oriented yarn, speeds of about 2500 to 4000 meters per minute are employed with a draw ratio of about 1.5–2.0, resulting in a final wind-up of about 6000 meters per minute. Peacock goes on to describe highly oriented yarns that can be produced from spinning speeds of up to 6000 meters per minute without further drawing.